This invention relates to the molding of plastic optical lenses, and in particular to the molding of plastic ophthalmic lenses.
Over the years many techniques have been developed for molding plastic lenses. Generally, these techniques involve arranging two circular lens mold elements, hereinafter called "half molds," in a parallel orientation to form a casting cavity therebetween. Thge half molds are generally made of glass and have surfaces which are selected to provide a desired curvature on either side of a finished lens. Liquid resin material is inserted between the two glass molds and cured. Depending on the resin involved, the curing can take place through the use of a catalyst and/or raising the temperature of the resin. One well-known polymer which provides excellent results for production of plastic lenses is a polycarbonate produced from allyl digycol carbonate and sold under the trademark CR39.
When the CR39 resin is used, curing is generally done at an elevated temperature. Curing techniques for the resin are well known in the art. Prior U.S. patents showing the use of two glass mold halves to form a molding cavity for a plastic resin are U.S. Pat. No. 4,190,621 issued to Greshes on Feb. 26, 1980 and U.S. Pat. No. 4,227,673 issued to Goodwin, et al on Oct. 14, 1980. The subject matter of these patents is hereby incorporated by reference.
In filling a prescription for an ophthalmic lens, there are ordinarily two types of corrections which are routinely made. One is magnification or minification depending on whether a given person is farsighted or nearsighted. This correction is usually made by providing curves on both sides of the lens. Each of these curves is symmetrical about the central axis of the lens. For this reason the magnification and minification corrections can be made without regard to orientation of the two halves of the mold.
The other correction is for astigmatism. This correction usually involves creating a slight cylindrical cut into the lens along an axis which is substantially parallel to the surface of the lens and passes through the central axis. In order to properly correct an individual's astigmatism the astigmatic axis must be oriented at the particular angle with respect to the individual's eye which the doctor has prescribed. Thus, the lens mold half which carries an astigmatism correction will usually have its astigmatic axis marked thereon. This axis should also be noted on the lens itself so that when the lens is cut for putting into a pair of frames the axis is at the proper angle.
Where a magnification or minification correction along with an astigmatism correction are the only corrections to be made by the opthalmic lens being cast, there is no need for any specific orientation of the mold half carrying the astigmatism correction with respect to the other mold half. All that needs to be done is to note where the astigmatism correction axis is on the lens so that when the lens is cut for a given pair of eyeglass frames the axis is at the correct angle with respect to the wearer's eyes.
However, where a bifocal is to be included in the lens being cast, then the astigmatism axis must be oriented properly with respect to the bifocal segment. In order to provide the proper arrangement of the astigmatism correction axis with respect to the bifocal segment, it has been found convenient to have protractor style index markings directly on the lens casting mold so that the axis line marked on the glass half mold carrying the astigmatism correction can be properly oriented. This problem is dicussed in detail in the Greshes patent.
It is desirable to provide the index markings at a location on the lens casting mold that is adjacent the location of the half mold carrying the astigmatism correction. However, when a flexible gasket is used, it has been found that the markings cannot conveniently be made on the interior surface of the flexible gasket because they tend to interfere with the insertion and holding of the half mold. If the markings are placed only on the outer surface of the gasket they must be viewed through the inner surface and are subject to some parallax error due to the diference in index of refraction between the ambient air and the gasket material. Further, if the markings are placed only on one of the ends of the gasket, some errors are likely to occur due to the fact that the markings are not directly adjacent the mold half carrying the astigmatism correction.
Although there are other ways to cast lenses, such as that disclosed by the Greshes patent, the most common method now being used is to seal the cavity between the two mold halves using a cylindrical gasket. The cylindrical gasket is often referred to as a "T-type" gasket because in cross-section it has an inner annular shoulder approximately half way between each end of the cylindrical gasket. The sides of the shoulder are intended to seal the edges of each of two opposing, circular glass mold halves. Sealing the edges of the casting cavity is important because exposure to air can cause changes in the cure rate of the portion of the resin which is exposed to the air. Thus, in order for the resin to harden uniformly it is preferable to have as little exposure to outside air as possible. Further, during the curing process of the resin, the newly forming lens will actually shrink a substantial amount. For the resin, CR39, the shrinkage is approximately 14%. Thus, there must be some flexibility in the shoulder to allow the two mold halves to approach each other somewhat while maintaining the seal.
Another critical concern in casting plastic lenses using flexible gaskets is how to fill the cavity between the two mold halves so that a minimum of air is trapped in the resin.